This invention relates to semiconductor memories, and particularly to a memory cell using merged transistors and capable of organization in a random access read-write memory of reduced size.
In the present state of the art merged transistors are employed in a read-write random access memory cell of such reduced size that bipolar arrays approaching the packing density of MOS storage devices may yet be attained. The key to such compact bipolar memory cells is the use of current injection transistors which replace the usual resistive loads and which feed current to inversely operated, or collector-up, flip-flop transistors. A compact bipolar memory cell of such kind is described in an article by Siegfried K. Wiedman and Horst H. Berger, entitled "Superintegrated Memory Shares Functions on Diffused Islands" published in Electronics, Feb. 14, 1972, pp. 83 - 86.
One drawback of the memory cell described in the above referenced article is that it requires two coupling transistors to provide access to the flip-flop or bistable device comprising the two pairs of merged transistors. Thus, there is required two coupling transistors, two current injection transistors, and two inversely operated flip-flop transistors, for a total of six transistors. In addition, there are four lines required to access the cell including power lines. It would be desirable to reduce both the number of transistors and the number of access lines to achieve an even smaller cell.
Another drawback of the above-referenced cell is that for large arrays, such as for 4K and higher number of bits, the operation is such that it is possible to write information into a selected cell during a time when information is being read. This anomalous condition may occur in a large array having a large number of coupling transistors coupling the storage cell to the read-write lines, because the coupling transistors may draw so much current from the read-write lines as to cause the selected cell to flip to its other state. This situation can be explained further by noting that during the read operation, the selected cell is sourcing current to the read-write lines, while the non-selected cells are sinking current from the same lines. In a small array the sourcing current of the selected cell with predominate over the sinking current of the non-selected cells, but in a large array it is likely that the sinking current of the non-selected cells will predominate over the sourcing current of the selected cell and cause the selected cell to be flipped to its other state, which is a write operation.